Note that study recruitment was designed so the final sample would be representative of the actual U.S. population and 2,381 out of 3,000 starting the study completed it.

Some cities and states have considered requiring a cigarette pack-like health warning on sugary drinks, and now there is evidence, albeit limited and preliminary, that such labels could have the desired effect.

Researchers randomly assigned more than 2,000 parents of young children, recruited through an online survey, to view one of six types of images of soda containers. One had no warning label; another showed only the caloric content; and four text labels that, with slight variations, warned about the risks of obesity, diabetes, and tooth decay in plain block letters on a white background.

About 40% of parents chose a sugar-sweetened beverage (SSB) for their child when it showed one of the warning labels, compared to 60% for no label and 53% with the caloric label, according to Christina Roberto, PhD, at the T.H. Chan School of Public Health at Harvard, and colleagues writing in Pediatrics.

Parents in the warning label groups also reported wanting to receive fewer SSB discount coupons and believing that SSBs were less healthy than other options for their child, and said they were less likely to buy SSBs in the future.

"Any strategy that can reduce their intake is thus, in my opinion, desirable," wrote George Bray, MD, of the Pennington Biomedical Research Center in Louisiana, in an email. "The inclusion of calorie information is good, but this article makes clear that more health related messages reduce the likelihood that people will purchase these beverages even more." Bray was not involved with the study.

Last year, San Francisco passed a law that all advertisements for sugary drinks must have a warning label. But the law hasn't been implemented yet because of an ongoing lawsuit, Roberto and colleagues noted.

In the study, participants were told to choose a beverage for their child from a virtual vending machine, and they rated their perceptions of different beverages and indicated whether or not they'd be interested in receiving beverage coupons. They were shown 20 popular, 20-ounce beverages, 12 of which contained sugar, with randomization to the six test conditions.

Roberto and colleagues swapped "obesity" for "weight gain" in one version of the warnings because they thought that weight gain might be more "influential than obesity" for parents. The differences in labels had only a small effect on purchases and were "unlikely to be consequential," wrote the authors.

Those placed in the warning label group reported that SSBs had more added sugar than other drinks and said they were less likely to buy them for their kids.

All participants were recruited through a sampling firm and reported being the primary caregiver for a child ages 6 to 11. The firm recruited the participants via social media, text messages, online panels, and other virtual communities, and offered a variety of incentives, including cash, prizes, sweepstakes, or the option to donate to charity.

Recruitment was patterned so that the final sample would be representative of the actual U.S. population based on 2010 census data. More than 3,000 participants started the study, with 2,381 completing it.

An important limitation to the study was that, when a warning label appeared on the SSBs, it was enlarged and displayed right above the image of the drink. "Because this is among the first studies on SSB warning labels, we tested labels under conditions when they were highly visible and salient," the authors explained. "If no effects are detected, there is little reason to think warning labels would work better in the real world."

Education level mattered in only two of of the 16 outcomes that participants were asked about: those who were less educated had significantly higher calorie estimates for SSBs and perceived the risk of type 2 diabetes to be higher with the drinks, compared with more educated participants.

Nearly three-quarters of the participants said they were in favor of an SSB warning label, and those numbers didn't vary significantly based on reported political party.

Besides the extra-prominent label display, other limitations included a possibility that participants reported not wanting SSBs because they thought the answer was desirable. In addition, the authors couldn't say how those who opted to take the survey might have differed from those who didn't, thus limiting the generalizability of the findings.

"Although this study provides preliminary support for placing warning labels on SSBs, more research is needed to understand how they would influence a range of consumers and whether they would impact overall dietary choices," Roberto and colleagues wrote. "For example, one concern is that warning labels on SSBs would be ineffective at reducing overconsumption of calories and sugar because people would simply compensate by buying other high sugar foods that are unlabeled."

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